The present invention generally relates to a cylinder head construction for an internal combustion engine and, more particularly, to the cylinder head construction for a multicylinder DOHC (double overhead camshaft) automobile engine of a type having spark plugs arranged in line with each other and generally in alignment with longitudinal axes of respective combustion chambers with camshafts positioned on respective sides of the row of the spark plugs.
The multicylinder DOHC automobile engine is not a recent development and is well known in the art. In general, the multicylinder DOHC engine employs a cylinder head supporting a pair of spaced camshafts and carrying spark plugs in a row intermediate of the space bewteen the camshafts and generally in concentric relationship with the respective combustion chambers. For avoiding contamination of each of the spark plugs with lubricant oil applied to various movable parts of a valve drive mechanism inlcuding the associated overhead crankshaft, it is known to surround each spark plug with a tubular member such as shown in, for example, Japanese Utility Model Publication No. 47-24533 published Aug. 2, 1972; British Patent Specifications No. 1,032,927 and No. 1,256,401, published June 15, 1966 and Dec. 8, 1971, respectively; and U.S. Pat. Nos. 3,520,286, 3,908,606 and 4,186,706, issued July 14, 1970, Sept. 30, 1975 and Feb. 5, 1980, respectively. However, of these publications, only British Patent Specification No. 1,256,401 positively describes the details of the tubular member for each spark plug and, according to it, the tubular member comprises a thin-walled tube separate from, but pressure fitted into a corresponding spark plug well of, the cylinder head.
Regardless of whether the tubular member is separate from the cylinder head construction or whether it is integrally formed with, and hence, an integral part of, the cylinder head construction, the tubular members one for each spark plug disclosed and suggested in any one of the above mentioned publications project upwardly from an upper deck of the cylinder head generally independently from each other. This, and a problem inherent therein, will be discussed with particular reference to FIG. 1 of the accompanying drawings, which illustrates one example of a prior art cylinder head in longitudinal sectional representation.
The prior art cylinder head shown therein has upper and lower decks 2 and 3, front and rear walls and a pair of opposite side walls, the walls being integrally formed with each other and also with the lower deck 3 to provide a generally rectangular container-like configuration. The lower deck 3 has portions each defining a top wall of the associated combustion chamber 4 and carries spark plugs 5 equal in number to the combustion chambers in the engine. A space between the upper and lower decks 2 and 3 provides a coolant passage 10 forming a part of any known engine cooling system. The tubular members, one for each spark plug 5, are generally identified by 7 and extend upwardly from the upper deck 2 so as to surround the respective spark plugs 5.
In this prior art cylinder head, as a matter of fact, the lower deck 3 is apt to be heated to a temperature much higher than the upper deck 2 is heated, even though the cylinder head is forcibly cooled by the flow of the cooling water. This is because the lower deck 3 is partially exposed direct to heat eveolved by the combustion of air-fuel mixture taking place cyclically in the combustion chamber. This differentiated heating of the upper and lower decks permits the lower deck to undergo a thermal expansion more readily than the upper deck and, therefore, the cylinder head as a whole tends to warp in a direction longitudinally thereof as shown by the phantom line in FIG. 1.
Once the tendency of the cylinder head to warp occurs, and unless the cylinder head is firmly bolted to the cylinder block, front and rear ends of the cylinder head will separate from corresonding end portions of the cylinder block with gases inside the adjacent combustion chambers consequently permitted to partially leak to the outside. Moreover, the longitudinal warp of the cylinder head results in displacement of some of the camshaft bearings relative to the others thereof, thereby hampering a smooth rotation of the camshafts.